This invention is directed to a novel thermoplastic copolyester elastomer composition that is modified by certain alkali metal salts of aliphatic polycarboxylic acids.
Thermoplastic copolyester elastomers form a highly useful class of polymers because of their outstanding physical properties. However, it is known that copolyester elastomer compositions generally are not suitable for blow molding because of their low melt strengths. Furthermore, copolyetheresters containing ethylene terephthalate ester units are known to process poorly because of their slow hardening rate. This problem of hardening rate also exists for soft copolyetheresters containing a low percentage of butylene terephthalate ester units.
To attempt to overcome these problems, a variety of compounds have been added to the copolyester elastomers. For example, the sodium salt of a monocarboxylic acid, such as sodium stearate, can be added to the polymer to increase melt strength and also to increase hardening rate. However, the melt stability of these compositions at processing temperatures is adversely affected, as indicated by a rapid increase in melt index. It is also known to use salts of polymeric carboxylic acids, such as ethylene/methacrylate acid ionomers, as agents to improve the melt strength of copolyesters. These materials serve as nucleating agents in copolyetheresters with no detrimental effect on melt stability. However, copolyester blends with the ionomer have been found to exhibit phase separation, resulting in the formation of layers when the blend is used in high shear processing techniques such as injection molding or injection blow molding. The resulting layers separate from one another (delaminate) when such nonhomogeneous articles are flexed or stretched, for example. Obviously, such performance is generally unacceptable.
The present invention describes a copolyester elastomer composition that overcomes the above-mentioned deficiencies. The novel composition is blow moldable, melt stable, and hardens rapidly after it is shaped. It does not exhibit phase separation in high shear processing techniques including injection molding and injection blow molding.